The aim of this study is to analyse the differences between physical fitness, the practise of physical activity (PA) and the academic performance of students of 4th grade of Secondary School. These differences will be analysed based on their gender and their performed physical activity. 156 students took part in this study (15.34 +/- .58 age, 1.69 +/- .09 m, 58.97 +/- 9.36 kg, 20.51 +/- 2.10 kg/ m2), 56 of these students being male and 77 of them being female. The survey respondents' physical fitness was evaluated by physical tests. PA was analysed by a different questionnaires (Enkid, Fitnessgram, Pace, or the PAQ-A) about the teenagers' physical activity habits. AP was evaluated using the arithmetic mean from the students' high school academic records. The results obtained in this study showed that girls and students that practise non-competitive PA obtain worse levels (p <.05) of physical fitness in every analysed quality except flexibility, but their AP reach higher levels. Nevertheless, boys and students who practise competitive PA obtain better physical fitness and worse AP. To sum up, we can generally observe that boys and students who perform competitive PA show a higher frequency of PA performance during the afternoon. Simultaneously, they obtain better results in physical fitness tests, even though their AP is worse.